In recent years, in various technical fields, global environment problems including global warming have become a matter of increasing concern. Under such a situation, in power-generating plants, it is urgent to suppress the total emissions of carbon dioxide gas, and in a plant to be built newly, the equipment is strongly required to be capable of producing electric power with high efficiency. For example, in thermal power generation boilers, the temperature and pressure of steam is made high as effective measures for producing electric power with high efficiency. This high-temperature and high-pressure steam causes a rise in tube wall temperature of superheater tubes and reheater tubes for boilers, and a steel pipe for boilers that is used therefor is required to have high-temperature strength and resistance to high-temperature oxidation caused by steam. Concerning a method of preventing steam oxidation of steel pipe, various proposals have been made so far as described below.
(A) Technique in which Solution Heat Treatment is Performed after Worked Layer has Been Formed
Patent Document 1 discloses an invention in which after an austenitic stainless steel pipe has been subjected to solution treatment, the pipe surface is subjected to cold working such as shot working, grinding, or polishing, and then the steel pipe is subjected to predetermined re-solution treatment. Patent Document 2 discloses an invention in which an austenitic stainless steel pipe is cold worked at a working ratio of 20% or higher, and then is subjected to solid solution heat treatment at a temperature rise rate of 2.9° C./sec or lower.
Patent Document 3 discloses an invention in which on the inner surface of an austenitic iron alloy pipe, a fine grain layer having a thickness of 30 μm or larger is formed by using grains finer than No. 7 grain size number, and thereafter the iron alloy pipe is cold worked at a working ratio of 20% or higher, and then is subjected to recrystallization treatment. Patent Document 4 discloses an invention in which an austenitic stainless steel pipe is cold worked so that the hardness at a position 20 μm distant from the inner surface thereof is Hv320 or higher, and is subjected to solution treatment.
(B) Technique in which a Fine Grain Structure is Kept Even after Solution Heat Treatment by Increasing the Contents of C and N
Patent Document 5 discloses an invention relating to an austenitic stainless steel pipe that has a fine grain structure of the grain size number of No. 7 or higher on the inner surface thereof, and contains 0.15% or higher of (C+N) in the fine grain layer.
(C) Technique for Forming Cold Worked Layer by Peening
Patent Document 6 discloses an invention in which after an austenitic stainless steel pipe has been finally heat treated, the steel pipe is subjected to peening by blasting particles onto the inner surface of the steel pipe. Patent Document 7 discloses an invention in which an austenitic stainless steel pipe is peened under predetermined conditions to form a worked layer having a thickness of 10 μm or larger. Patent Document 8 discloses an invention in which a tube body taken out of an existing boiler is subjected to chemical cleaning to remove scale from the inner surface thereof after being heat treated, and thereafter the inner surface of the tube body is subjected to shotblasting to form a cold worked layer.
(D) Technique for Improving Adhesiveness of Scale
Patent Document 9 discloses an invention relating to a steel pipe for boilers excellent in steam oxidation resistance, which is produced by subjecting an austenitic stainless steel pipe containing a rare earth metal to solution treatment, and is provided with a peened layer by blasting particles onto the inner surface of the steel pipe. Patent Document 10 discloses an invention relating to a steel pipe in which 9 to 28 mass % of Cr is contained, the maximum height of the inner surface after cold working is 15 μm or larger, and further the difference in Vickers hardness between the inner surface layer and the central portion of wall thickness of pipe is 100 or more.
(E) Technique for Subjecting Steel Pipe to Cold Working of High Working Ratio
Patent Document 11 discloses an invention in which the inner surface of a heat-resistant ferritic steel pipe or a heat-resistant austenitic steel pipe containing 5 to 30 mass % of Cr is subjected to ultrasonic impact treatment. Patent Document 12 discloses an invention relating to an austenitic stainless steel pipe for boilers which contains 16 to 20 wt % of Cr and the inner surface of which is cold worked, wherein the Cr concentration at a position in the vicinity of the inner surface of steel pipe is 14 wt % or higher, and the hardness at a position 100 μm distant from the inner surface of steel pipe is 1.5 times or more the average hardness of the base metal or Hv300 or higher. Patent Document 13 discloses an invention relating to a steel pipe excellent in steam oxidation resistance, which contains 8 to 28 mass % of Cr and has a hard worked layer.
(F) Technique for Improving Steam Oxidation Resistance of Heat-Resistant Ferritic Steel
Patent Document 14 discloses an invention relating to a method for working a heat-resistant ferritic steel in which a steel containing 9.5 to 15% of Cr is normalized and tempered to uniformize the crystal grains and steel micro-structure, and thereafter a shot worked layer is formed by blasting particles onto the surface of steel.